THE FUEL OF THE SUN. 17 



wise night would be as light and as warm as day. What 

 becomes of the enormous force thus apparently non-recur- 

 rent in the same form? 



This is a grand question, a philosophical thought worthy 

 of the author of ' ' The Correlation of Physical Forces." 

 Most philosophical thinkers will, I believe, agree with me in 

 concluding that a sound reply to it will solve the great mys- 

 tery of the everlasting radiations of our sun and all the other 

 suns of the universe. So long as we regard these suns as 

 the sources of continually expended forces of light and heat, 

 their everlasting and unabated renewal becomes a mystery 

 utterly inscrutable to the human intellect, since the creation 

 of new force, or any addition to the total forces of the 

 universe, is as inconceivable to us as any addition to the 

 total matter of the universe. The great solar question as- 

 sumes a far more hopeful shape when we admit that all the 

 forces of past radiations are somewhere diffused in space, 

 and we ask whether a sun contains any mechanism by 

 which it may collect and concentrate this diffused force, and 

 thus perpetually gather from surrounding suns as much as 

 it radiates towards them. 



The next part of my work is an attempt to show that such 

 a mechanism does exist in our solar system, and to explain 

 its action. 



Ws know that if atmospheric air is compressed it becomes 

 heated, that if this heat is allowed to radiate and the air is 

 again expanded to its original dimensions, it will be cooled 

 below its original temperature to an extent precisely equal 

 to the heat which it gave out when compressed. On this 

 principle I endeavor to explain the everlasting maintenance 

 of the solar and stellar radiations. 



The sun is attended by his train of planets whose orbital 

 motion he controls, but they in return react upon him as 

 the moon does upon the earth. If this reaction were reg- 

 ular, like that of the moon upon the earth, a regular at- 

 mospheric tide would result ; but the great irregularity of 

 the dimensions, distances, and velocities of the planets pro- 

 duces a result equivalent to a number of clashing irregular 

 tides in the solar atmosphere ; or, otherwise stated, the 

 centre of motion and centre of gravity of the whole system 



